Cardiac rhythm management devices are implantable devices that provide electrical stimulation to selected chambers of the heart in order to treat disorders of cardiac rhythm. A pacemaker, for example, is a cardiac rhythm management device that paces the heart with timed pacing pulses. The most common condition for which pacemakers have been used is in the treatment of bradycardia, where the ventricular rate is too slow. Atrio-ventricular conduction defects (i.e., AV block) that are permanent or intermittent and sick sinus syndrome represent the most common causes of bradycardia for which permanent pacing may be indicated. If functioning properly, the pacemaker makes up for the heart's inability to pace itself at an appropriate rhythm in order to meet metabolic demand by enforcing a minimum heart rate and/or artificially restoring AV conduction.
In pacemaker patients who are chronotropically incompetent (e.g., sinus node dysfunction), the heart rate is determined solely by the pacemaker in the absence of intrinsic cardiac activity. That heart rate is determined by the programmed escape intervals of the pacemaker which cause paces to be delivered to the atria and/or ventricles, depending upon the pacing mode, if no intrinsic beats occur before expiration of the escape intervals. Pacing the heart at a fixed rate as determined by the length of the programmed escape intervals, however, does not allow the heart rate to increase with increased metabolic demand. It is for this reason that rate-adaptive pacemakers have been developed which vary the programmed escape intervals in accordance with one or more physiological parameters related to metabolic demand such as obtained from an accelerometer or minute ventilation sensor. In chronotropically competent patients in need of ventricular pacing, on the other hand, atrial triggered pacing modes such as DDD or VDD are desirable because they allow the pacing to track the physiologically normal atrial rhythm, which causes cardiac output to be responsive to the metabolic needs of the body. For this latter group of patients, the pacemaker is normally programmed so that the atrial rate is overridden by an atrial or ventricular pace only if the atrial rate drops to a level considered unsafe.
A change in a patient's chronotropic status from competent to incompetent after initial implantation of a cardiac rhythm management device is usually not addressed until the patient has become very symptomatic and has gone through a formal clinical evaluation via exercise testing. Only then is the patient provided with additional therapy such as reprogramming of the pacemaker to deliver rate-adaptive pacing. The present disclosure is directed toward ways of improving this situation.